Step 1

If using frozen blueberries, thaw 5 cups (about 24 ounces). Cut 1 stick unsalted butter into a few large pieces and let sit at room temperature until softened.

Step 2

Arrange a rack in the middle of the oven and heat the oven to 375°F. Coat an 8- or 9-inch square or other 2- to 2 1/2-quart baking dish with butter.

Step 3

Finely grate the zest of 1/2 small lemon into a medium bowl (about 1 teaspoon); reserve the zested lemon for the filling. Add 1 cup all-purpose flour, 1/2 cup packed light or dark brown sugar, 1 teaspoon baking powder, and 1/4 teaspoon kosher salt, and whisk to combine.

Step 4

Add the butter and toss and work with your fingers into the dry ingredients until large, heavy crumbles are formed and no loose flour remains.

Step 5

Drain the liquid from the thawed frozen blueberries.

Step 6

Place 1/2 cup granulated sugar, 1/4 cup cornstarch, 1/2 teaspoon ground coriander, and 1/2 teaspoon kosher salt in the baking dish and whisk to combine and break up any lumps.

Step 7

Add the blueberries and 1 teaspoon vanilla extract. Juice the zested lemon until you have 1 tablespoon juice, then add to the baking dish. Toss gently to combine, then spread into an even layer.

Step 8

Break the crumble topping into large clumps with your fingers and arrange over the filling in an even layer. Sprinkle with 1/4 cup sliced almonds if using.

Step 9

Bake until the crumble topping is golden-brown and the filling is bubbling, 30 to 35 minutes. Let cool for at least 15 minutes before serving. Top with vanilla ice cream if desired.